1. Field of the Invention
The present invention .relates to a deflection yoke of a self-convergence system, which is fitted to an in-line type color-picture tube, and particularly related to providing a deflection yoke capable of correcting an inverted pattern of cross misconvergence easily.
2. Description of Prior Art
The deflection yoke of a self-convergence system has been employed as one method to provide good convergence of three electron beams emitted from three electron guns on the screen of an image display using a three-electron gun, in-line color-picture tube.
This type of deflection yoke is designed to obtain good beam convergence by forming horizontal- and vertical-deflection magnetic fields into a strong pincushion shape and a strong barrel shape, respectively, by using saddle-type horizontal deflection coils and saddle-type vertical deflection coils.
However, since the deflection angle may become as large as approximately 90 degrees on the screen of a color-picture tube, the above-mentioned magnetic field distribution originally designed to provide good convergence generates distortions in the shapes of the pincushion and barrel in the upper and lower rasters of the screen, and therefore cannot be put into practical use. On the other hand, correction of these raster distortions introduces cross misconvergenee as shown in FIG. 5 and FIG. 6, leading to the same problem. Thus, it has been very difficult to simultaneously satisfy the requirements of correcting the distortions of the upper and lower rasters and obtaining good convergence. In order to solve the aforementioned problem, a method which conventionally employs a saturable reactor or the like is used in the deflection circuit. Although this method can almost completely eliminate cross misconvergence, it generates an "inverted pattern of cross misconvergence" such that negative and positive cross misconvergences occur respectively in the peripheral regions (a) and middle regions (b) of the screen, as shown in FIG. 7, giving no satisfactory result for displays requiring high precision.
In addition, when this method is applied to flat-face picture tubes with a small degree of screen curvature, it has sometimes been a problem even for general purpose tubes not requiring high precision, because the above-mentioned inverted pattern is emphasized more.
Furthermore, the conventional practice of correcting the inverted pattern has relied on the method where each deflection yoke is corrected by manually adding magnetic pieces or the like one by one, which results in low production efficiency.
The inverted pattern of FIG. 7 reveals that, with the conventional method, the vertical deflection magnetic field shifts toward the above pincushion type magnetic field in the middle region (b) of the screen and toward the above barrel-type magnetic field in the peripheral region (a) of the screen compared with the best magnetic field distribution an ideal magnetic field distribution simultaneously satisfying both the distortion correction of the upper and lower rasters, and convergence.